1. Field of the Invention
The present invention relates to an inverter circuit driving device of an induction heating rice cooker, and more particularly to an inverter circuit of an induction heating rice cooker which is capable of varying the width of a drive pulse to an inverter with a variation in an input voltage so as to vary a switching frequency of the inverter, thereby stabilizing heating power generated by a switching operation of the inverter and preventing an internal device of the inverter from being damaged due to the variation in the input voltage.
2. Description of the Related Art
FIG. 1 is a sectional view of a general induction heating rice cooker. As shown in this drawing, the rice cooker comprises a body 1, an inner pan 2 disposed inside of the body 1 for containing an object to be cooked, and a cooking heater 3 mounted in a portion of the body 1 beneath the inner pan 2 or an inner suffice of the body 1 for cooking the object contained in the inner pan 2.
This rice cooker is a home appliance which heats and cooks the object contained therein at more than a predetermined temperature, and is mainly used to boil rice. It is also used to retain warmth of food for a long time. A user puts rice or other additive food and an appropriate amount of water in the inner pan 2 and, in turn, the inner pan 2 in the body 1. The user then sets an amount to be cooked and a cooking mode and enters a cooking command containing the settings to the rice cooker. In order to automatically cook in response to the cooking command, the rice cooker comprises a controller for controlling a cooking operation of the cooker.
The rice cooker is of a specific type based on how cooking heat is provided to the inner pan 2. For example, an induction coil having a plurality of uniformly-spaced turns is formed in a portion of the cooker body 1 receiving the inner pan 2, and induced current is generated in the inner pan 2, which is made of a magnetic substance, due to a magnetic field resulting from the flow of current in the coil, so as to heat the inner pan 2. This type of rice cooker is a so-called induction heating rice cooker.
FIG. 2 is a block diagram showing the construction of a conventional inverter circuit of the induction heating rice cooker.
The inverter circuit is applied to the induction heating rice cooker to induction-heat an object (load) to be cooked by controlling a power switching device therein. The power switching device performs a switching operation in response to a control signal to apply a drive voltage to the induction coil for the inner pan so as to heat the inner pan. The inverter circuit comprises a power source 10 for supplying a commercial alternating current (AC) voltage, a rectifier 20 for rectifying the AC voltage supplied by the power source 10, a filter 30 for filtering an output voltage from the rectifier 20, and an inverter 40 for performing a switching operation based on an output voltage from the filter 30 to apply a drive voltage to the induction coil for the inner pan.
The inverter circuit further comprises a trigger circuit 50 for varying the operation or state of a specific circuit at a rising or falling edge of a pulse. The trigger circuit 50 generates a drive pulse to drive a switch of the inverter 40. The switch is turned on when the drive pulse goes “high” in level, and off when the drive pulse goes “low” in level. Here, a voltage applied across the switch is referred to as a “switched voltage”.
The inverter circuit further comprises a switch driver 60 for transferring the drive pulse from the trigger circuit 50 to the inverter 40 to drive the switch of the inverter 40.
The switch of the inverter 40 is adapted to perform the switching operation in response to the drive pulse transferred from the switch driver 60 to generate heating power, which is used as a heating source for cooking a load to be heated (an object to be cooked). A withstand voltage of the switch against the switched voltage is limited in its range according to specifications of the switch. In general terms, the higher the withstand voltage, the higher the cost of the switch, resulting in an increase in production cost.
Note that a commercial AC voltage from a power plant may have input/output characteristics varying with time zones/regions or be unstably supplied due to noise. In this case, the varying AC voltage is supplied as a drive voltage to the inverter 40, which then performs the switching operation at an abnormal frequency resulting from the unstable drive voltage, causing a variation in the heating power. As a result, even though a user enters the same cooking command with respect to the same object to be cooked, he/she cannot obtain a desired cooking result because of a variation in the heating power, resulting in the inconvenience of use and unreliability of the product.
Particularly, in the case where the AC voltage increases abruptly, the switched voltage across the switch may exceed the withstand voltage of the switch, thereby damaging the switch and reducing durability of the product.
In order to prevent the above problem, the conventional inverter circuit additionally includes a separate current transformer for sensing the amount of current flowing to the switch, or a separate protection circuit for sensing the amount of current flowing to the inverter 40 or the level of a voltage applied thereto, thereby increasing a production cost and, thus, an economic burden on a consumer.
FIG. 3 is a graph illustrating an input voltage-to-switched voltage relation of the conventional inverter circuit, and FIG. 4 is a graph illustrating a constant-power control method of the conventional inverter circuit. A description will hereinafter be given of characteristics of the conventional inverter circuit with reference to FIGS. 3 and 4.
Referring first to FIG. 3, the reference numeral G1 denotes a waveform of an input voltage to the inverter, and G2 denotes a waveform of a switched voltage applied across the switch. In this graph, the X-axis represents time and the Y-axis represents a voltage level. A commercial AC voltage of 220V-60 Hz is passed through the rectifier and filter and then applied as an input voltage of 220V-120 Hz to the inverter. The waveform G1 corresponds to one period of the input voltage to the inverter.
The switch performs the switching operation for each period of the input voltage corresponding to the waveform G1 in response to the drive pulse from the trigger circuit and thus generates a switched voltage which is higher in level than the input voltage.
At this time, the switched voltage across the switch must not exceed the withstand voltage of the switch so as to guarantee the normal switching operation without damaging the switch. However, seeing the waveform G2 of the switched voltage, a portion of the waveform G2 corresponding to ½ of a period of the switched voltage and portions in the vicinity thereof form a ridge of a sinusoidal wave, and the switched voltage may exceed the withstand voltage of the switch due to its abrupt increase in those portions.
The constant-power control method of the conventional inverter circuit, illustrated in the graph of FIG. 4, is employed in the induction heating rice cooker to maintain the heating power from the inverter 40 at a constant level.
In FIG. 4, the reference numeral {circle around (2)} denotes a drive pulse for the switching operation of the inverter in the normal state, {circle around (2)}′ denotes switched current flowing through the switch when the drive pulse is “high” in level, and {circle around (2)}″ denotes a switched voltage applied across the switch by the switched current. The drive pulse {circle around (2)}, switched current {circle around (2)}′ and switched voltage {circle around (2)}″ in the normal state are indicated by solid lines.
Where a variation or noise occurs in the input voltage, the inverter circuit performs a constant-power control operation by adjusting a turn-on period of time of the drive pulse from the trigger circuit. That is, in order to lower the heating power from the inverter, the inverter circuit reduces a high-level width of the drive pulse, as indicated by {circle around (1)}, by shortening the turn-on time period of the drive pulse. As a result, the amount of the switched current flowing through the switch is reduced as indicated by {circle around (1)}′ and the switched voltage applied across the switch is thus lowered as indicated by {circle around (1)}″. Consequently, the heating power is lowered.
On the other hand, in order to raise the heating power from the inverter, the inverter circuit increases the high-level width of the drive pulse, as indicated by {circle around (3)}, by lengthening the turn-on time period of the drive pulse. As a result, the amount of the switched current flowing through the switch is increased as indicated by {circle around (3)}′ and the switched voltage across the switch is thus raised as indicated by {circle around (3)}″. Consequently, the heating power is raised.
FIG. 5a is a graph illustrating a switching frequency-to-heating power relation of the conventional inverter circuit, and FIG. 5b is a graph illustrating a switching frequency-to-switched voltage relation of the conventional inverter circuit. A description will hereinafter be given of the problems with a conventional constant-power control method using a single frequency with reference to FIGS. 5a and 5b. 
It can be seen from FIG. 5a that the switching frequency and the heating power are in inverse proportion to each other. In this connection, it is preferred to make the switching frequency lower to raise the heating power in order to rapidly heat the load (inner pan).
It can be seen from FIG. 5b that the switching frequency and the switched voltage across the switch are in inverse proportion to each other. In this regard, if the switching frequency is made to be lower to raise the heating power on the basis of the relation of FIG. 5a, the switched voltage is raised rather on the basis of the relation of FIG. 5b, so it may exceed the withstand voltage of the switch, which leads to reductions in durability of the product and reliability of the induction heating rice cooker.
High heating power is generally required to shorten a heating period of time of the rice cooker. To this end, the switching operation of the inverter can be controlled under the condition that the switching frequency is fixed at A Hz. In this case, although the heating power of the rice cooker is raised as shown in FIG. 5a, the switched voltage is raised together so that it may exceed the withstand voltage of the switch and in turn damage the switch. In order to avoid this problem, a manufacturer may use a high-cost switch with a high withstand voltage instead of the above switch. However, the use of the high-cost switch increases a production cost and, thus, an economic burden on a consumer.
On the other hand, provided that the switching operation of the inverter is controlled under the condition that the switching frequency is fixed at B Hz, the switched voltage is lowered as shown in FIG. 5b, thereby making it possible to protect the switch. In this case, however, the heating power of the rice cooker is lowered, too, so the load (inner pan) to be heated cannot be rapidly heated. This reduces a heating efficiency and causes inconvenience to a user.